The present disclosure relates to receiver circuits, and more particularly to receiver circuits which frequency-convert high frequency signals used in mobile phones, broadcasting systems, etc. into baseband signals.
Generally, a high frequency signal input to a receiver circuit is amplified by a low noise amplifier (LNA) first, and then is frequency-converted into a baseband signal by a frequency converter. Then, after the baseband signal is amplified by an automatic gain control (AGC) amplifier, an analog filter performs low-pass filtering for attenuating the interference wave adjacent to the desired-wave band. In some cases, an equalizer is provided in a succeeding stage of the analog filter, and thus compensation is performed for a phase change due to a filtering operation of the analog filter. Some receiver circuits having such a configuration seek to reduce the power consumption by setting a relatively high cutoff frequency to the analog filter and by halting operation of the equalizer if no interference waves are present (see, e.g., Japanese Patent Publication No. 2008-005259).
In order to reduce the power consumption of a receiver circuit, reduction of the operating voltage is effective. However, reducing the operating voltage of a receiver circuit causes the distortion characteristic of each signal processing block, especially of the AGC amplifier which amplifies the baseband signal, to degrade significantly, thereby posing a problem in that the interference wave immunity of the receiver apparatus is reduced. That is, a conventional receiver circuit is not capable of sufficiently reducing a distortion component due to an interference wave only by reducing the operating voltage, and thus has limitations on the interference wave immunity. In addition, reducing the operating voltage of a receiver circuit also poses a problem in that if the reception level of the input desired wave is too high, the signal to noise plus distortion ratio (SNDR) is reduced due to a distortion component caused by the desired wave. This is a serious problem for a receiver circuit of an analog broadcast signal which requires a high SNDR such as an NTSC or PAL signal.